1. Field of the Invention
The present invention relates to a burner with staged fuel injection, which is composed of a swirl generator for a combustion air stream and means for the introduction of fuel into the combustion air stream, the means for the introduction of fuel into the combustion air stream comprising at least one first fuel supply with a first group of fuel outlet orifices and a second fuel supply with a second group of fuel outlet orifices downstream of the first group of fuel outlet orifices, and the first and second group of fuel outlet orifices and also inlet orifices for the combustion air stream being arranged along the swirl space formed by the swirl generator. A preferred field of use for a burner of this type is application in steam and gas turbine technology.
2. Brief Description of the Related Art
EP 0 321 809 B1 discloses a conical burner consisting of a plurality of shells, what is known as a double-cone burner. By means of the conical swirl generator which is composed of a plurality of shells and forms inside it a swirl space, a closed swirl flow in the conical head is generated, which, because of the increasing swirl along the cone vertex, becomes unstable and changes to an annular swirl flow with backflow in the core. The shells of the swirl generator are composed in such a way that tangential air inlet slits for combustion air are formed along the burner axis. At the inflow edge of the cone shells which occurs as a result, supplies for the premixing gas, that is to say the gaseous fuel, are provided, which have outlet orifices for the premixing gas which are distributed along the swirl space in the direction of the burner axis. The gas is injected through the outlet orifices or bores transversely to the air inlet gap. This injection leads, in conjunction with the swirl of the combustion-air/fuel-gas flow, said swirl being generated in the swirl space, to a good intermixing of the fuel gas or premixing gas with the combustion air. Good intermixing is the precondition, in premixing burners of this type, for low NOx values during the combustion operation.
For a further improvement in a burner of this type, EP 0 280 629 A2 discloses a burner for a heat generator, said burner having, after the swirl generator, an additional mixing zone for the further intermixing of fuel and combustion air. This mixing zone may be designed, for example, as a following tube, into which the flow emerging from the swirl generator is transferred without any appreciable flow losses. By means of this additional mixing zone, the degree of intermixing can be further increased and consequently the pollutant emissions can be reduced.
FIG. 1 shows diagrammatically an example of burners of this type, in which the fuel is mixed with the inflowing combustion air via outlet orifices in supply ducts arranged along the burner axis in the swirl body 1. The figure illustrates, in this case, the conical swirl body 1 of the burner, together with the swirl space 1a which is surrounded by said swirl body and along which run the fuel supplies, together with the outlet orifices 2, indicated in the figure by arrows for injected fuel. These fuel supplies are designed, as a rule, as individual ducts which have a fixed distribution of the fuel outlet orifices 2 along the burner axis. Furthermore, in FIG. 1, a piloting lance 5 can be seen, via which the fuel is injected directly into the swirl space 1a during the start-up of the burner. With increasing load, a changeover then takes place from this piloting stage to premixing operation, in which the fuel is intermixed with the inflowing combustion air via said fuel outlet orifices 2.
A further known burner geometry of a premixing burner is known from WO 93/17279. In this arrangement, a cylindrical swirl generator with an additional conical inner body is used. The premixing gas is injected into the swirl space likewise via supplies with corresponding outlet orifices which are arranged along the axially running air inlet slits. The piloting supply of this burner is provided at the end of the conical inner body. Piloting, however, leads to increased NOx emissions, since, in this operating mode, only insufficient intermixing with the combustion air can take place.
In all known burner systems, a single-stage supply of the fuel is provided in the premixing mode. The size, distribution, arrangement, spacing and number of the outlet orifices of the fuel supply along the burner axis must in this case be optimized in order to fulfil the requirements for low emissions, the extinction limit and the backflash limit and also the requirements for combustion stability. In this case, it is virtually impossible to fulfil all these requirements by means of a fixed distribution of the outlet orifices, even under changing operating and ambient conditions.
A further disadvantage of the known methods for the operation of premixing burners is that these are optimized for low emissions and low combustion oscillations under full-load conditions. In order to start up the burner and start the gas turbine, an additional piloting stage is required, which, however, causes the emission values to rise markedly.
In a parallel patent application of the applicant, to solve these problems, a burner arrangement was proposed, in which the means for the introduction of fuel into the combustion air stream comprise at least one first fuel supply with a first group of fuel outlet orifices for a first premixing fuel quantity and a second fuel supply with a second group of fuel outlet orifices, downstream of the first group of fuel outlet orifices, for a second premixing fuel quantity. The fuel supplies with the fuel outlet orifices are in this case arranged on the swirl body along the swirl space in the longitudinal direction of the burner and are subdivided into at least two ducts for the fuel which are independent of one another. The use of a burner system of this type with staged fuel injection makes it possible to have a markedly widened operating range, as compared with single-stage burner systems. In particular, the operating mode of the burner can be adapted optimally to the respective operating load in terms of the emissions. Furthermore, a fuel supply via the piloting lance is no longer necessary, since operating solely with fuel outlet orifices of the first stage (for example, 2a in FIG. 2) gives rise to sufficiently high local temperatures on the burner axis, while the overall adiabatic temperature continues to be low.
The extinction limit of a burner with a fully premixed fuel/air mixture has an extinction limit above 1600 K. Modern AAP gas turbines are operated, during idling and under low load, with a fuel/air mixture which, during combustion, generates an adiabatic temperature of 900 to 1600 Kelvin. It is therefore impossible to burn the fuel in all the available combustion air, so that it is necessary to enrich the core air in the burner by piloting in the region of the burner neck. This relates, in particular, to the abovementioned burners of the prior art with single-stage fuel injection.
The burners, developed by the applicant, with multistage fuel injection by the division of the fuel supplies into two separate regions show, in particular, a reduction in the NOx emissions and in the intensity of the combustion pulsations at higher flame temperatures above 1650 K. However, the problem of pulsations still arises at temperatures below 1500 K when the first stage is operated essentially alone and therefore performs a function similar to that of a piloting stage.
The object of the present invention is to provide a burner which generates a low level of pulsations even at lower combustion temperatures of below 1600 K.